Syrian activists say security forces have attacked rebels in the capital Damascus with helicopters and tanks, while rebels launched an offensive against the government in the northern city of Aleppo.

The activists say government troops used helicopter gunships and tank fire to try to drive rebels out of the Damascus neighborhoods of Barzeh and Mazzeh. Syrian state news agency SANA denied that helicopters were deployed and insisted the capital was “normal” as security forces chased what it called “terrorist” remnants from the streets.

Meanwhile, a rebel commander appeared in an Internet video, announcing that the battle to liberate the commercial hub of Aleppo has begun. Witnesses reported street battles in several districts including Salaheddine and Sakhour.

Elsewhere, Turkish officials said Syrian rebels seized another border crossing between the two countries on Sunday, taking control of the Bab al-Salama complex from Syrian troops. Rebels captured the Syrian side of another crossing (Bab al-Hawa) on the Turkish border last week.

But, Iraqi officials said Syrian insurgents withdrew from the Rabiya border crossing with Iraq overnight, allowing Syrian troops to reclaim it. Rebels remained in control of the Syrian side of the Albu Kamal crossing with Iraq.

Aleppo had been largely untouched by the 16-month uprising against Syrian President Bashar al-Assad. It is home to Syrian elites and merchants who have benefited from Mr. Assad's authoritarian rule, but recently has seen an increase in protests against his deadly crackdown on the rebellion.

Syrian state television broadcast an image of Mr. Assad meeting with his new army chief of staff. It said the Syrian president met General Ali Ayyoub on Sunday and gave him instructions. Mr. Assad promoted his previous army chief of staff to defense minister last week, after a Damascus bomb attack killed the sitting defense minister and three other top security officials.

Sunday's images of Mr. Assad were his second appearance on television since the attack. But, the Syrian president's whereabouts remain unknown and he has not spoken publicly in recent days, fueling perceptions that the assassination of four members of his inner circle has weakened his grip on power.